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The study, published Tuesday in the journal The Lancet Psychiatry, looked at disruptions in the circadian rhythms -- or daily sleep-wake cycles -- of over 91,000 adults in the United Kingdom. It measured these disruptions using a device called an accelerometer that is worn on the wrist and measures one's daily activity levels. The participants were taken from the UK Biobank, a large cohort of over half a million UK adults ages 37 to 73.

The researchers found that individuals with more circadian rhythm disruptions -- defined as increased activity at night, decreased activity during the day or both -- were significantly more likely to have symptoms consistent with bipolar disorder or major depression. They were also more likely to have decreased feelings of well-being and to have reduced cognitive functioning, based on a computer-generated reaction time test.

For all participants, activity levels were measured over a seven-day period in either 2013 or 2014, and mental health proxies such as mood and cognitive functioning were measured using an online mental health questionnaire that participants filled out in 2016 or 2017.

"It's widely known that a good night's sleep is a good thing for well-being and health. That's not a big surprise," said Dr. Daniel Smith, professor of psychiatry at the University of Glasgow and a leading author on the study. "But I think what's less well-known and what comes out of this work is that not only is a good night's sleep important, but having a regular rhythm of being active in daylight and inactive in darkness over time is important for mental well-being."

The findings were found to be consistent even when controlling for a number of influential factors including age, sex, lifestyle, education and body mass index, according to Smith.

"I think one of the striking things that we found was just the consistency in the direction of our association across everything we looked at in terms of mental health," Smith said.

Daily circadian rhythm is controlled by a collection of neurons in an area of the brain called the hypothalamus. The hypothalamus helps regulate a number of important behavioral and physiological functions such as body temperature, eating and drinking habits, emotional well-being and sleep, according to the National Institute of General Medical Sciences.

The findings are consistent with research indicating a link between sleep disruptions and mood disorders. A 2009 study, for example, showed that men who worked night shifts for four years or more were more likely to have anxiety and depression than those who work during the day.

However, the new study is the first to use objective measurements of daily activity and is among the largest of its kind, according to Aiden Doherty, senior research fellow at the University of Oxford, who was not involved in the research.

"This study is the first large-scale investigation of the association of objectively measured circadian rhythmicity with various mental health, well-being, personality and cognitive outcomes, with an unprecedented sample size of more than 90 000 participants," Doherty wrote in an email.

"Previous studies have been very small (in just a few hundred people), or relied on self-report measures (asking people what they think they do). ... However, this study used objective device-based measures in over 90,000 participants; and then linked this information to standard measures of mood disorders, subjective well-being, and cognitive function," he added.

The findings have significant public health consequences, particularly for those who live in urban areas, where circadian rhythms are often disrupted due to artificial light, according to Smith.

"By 2030, two-thirds of the world's population will be living in cities, and we know that living in an urban environment can be pretty toxic to your circadian system because of all the artificial light that you're exposed to," Smith said.

"So we need to think about ways to help people tune in to their natural rhythms of activity and sleeping more effectively. Hopefully, that will protect a lot of people from mood disorders."

For those who struggle to maintain a consistent circadian rhythm, certain strategies -- such as avoiding technology at night -- have proven to be an important part of good sleep hygiene.

"Not using your phone late at night and having a regular pattern of sleeping is really important," Smith said. "But equally important is a pattern of exposing yourself to sunshine and daylight in the morning and doing activity in the morning or midday so you can actually sleep properly."

Based on the observational nature of the study, the researchers were unable to show causality, meaning it is unclear whether the sleep disturbances caused the mental health problems or vice versa.

"It's a cross-sectional study, so we can't say anything about cause and effect or what came first, the mood disorder or the circadian disruption," said Kristen Knutson, associate professor of neurology at Northwestern's Feinberg School of Medicine, who was not involved in the study.

"And it's likely they affect each other in a circular fashion," she added.

The researchers also looked exclusively at adults between age 37 and 73, meaning the results may not apply to younger individuals, whose circadian rhythms are known to be different than those of older adults, according to Smith.

"The circadian system changes throughout life. If you've got kids, you know that very young kids tend to be nocturnal," Smith said. "My suspicion is that we might observe even more pronounced effects in younger samples, but that hasn't been done yet, to my knowledge."

But the study adds more credence to the idea that sleep hygiene -- including maintaining a consistent pattern of sleep and wake cycles -- may be an important component of good mental health, according to Smith.

"It's an exciting time for this kind of research because it's beginning to have some real-world applications," Smith said. "And from my point of view as a psychiatrist, I think it's probably under-recognized in psychiatry how important healthy circadian function is, but it's an area that we're trying to develop."

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